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Doubling & sample sets

Good morning, everyone. I am a G.I.A. student trying to pass the 20-stone test. I keep missing 1, so that’s a failing grade. Inevitably, the misidentified stone relies on the ability to see doubling (I.e., zircon vs. some other clear stone). I have tried every strategy for seeing doubling (such as using a polarized filter) and I still have trouble distinguishing doubling from facet reflections or the switch from light to dark with the filter. When I look at internet & G.I.A. photos/videos of doubling, it’s obvious, but not in my test stones (to me, anyway). Any suggestions?

Also, is there somewhere that I could get a variety of OTL clear stones on which to practice (also that won’t cost me the proverbial arm and a leg).
Thanks in advance for any help you can give.



You could try They have no resererve auctions and you should be able to find specimens there that you could come by pretty cheap.


you should be able to pick this doubling up looking through the crown in zircon reasonably easy as per this photo

Thank you, Pierre. I appreciate your quick and helpful response.

Be well,


Hi, Syd,

Thank you for the photo and suggestion. I have tried that technique more times than I can say, as well as using a polarized filter. I can clearly see the doubling in the photo, but I seem to still have problems looking at an actual stone from the test sets.

Be well,



thanks De, gemmology is not easy, if it was everyone would be doing it, most practical gemmology takes time and practice, same with refractometer, spectroscope, dichroscope, microscope. etc. if you are not doing it all the time you will lose the skills “keep practicing”

Thank you for the encouragement, SydP. I appreciate it and will keep plugging away at it.